The silicon based MEMS microphone, also known as an acoustic transducer, has been in research and development for many years. Because of its potential advantages in miniaturization, performance, reliability, environmental endurance, costs and mass production capability, the silicon based MEMS microphone is widely used in many applications, such as cell phones, hearing aids, smart toys and surveillance devices.
In general, a silicon based MEMS microphone consists of four elements: a fixed backplate, a highly compliant, moveable diaphragm (which together form the two plates of a variable air-gap condenser), a voltage bias source and a buffer. The two mechanical elements, the backplate and the diaphragm, are typically formed on a single silicon substrate. One of these two elements is generally formed to be planar with the surface of the supporting silicon wafer, and the other element, while itself generally planar, is supported several microns above the first element by spacer or sidewalls.
Patent application No. WO 02/15636 discloses an acoustic transducer. The acoustic transducer has a diaphragm positioned between a cover member and a substrate, and the diaphragm can be laterally movable within a plane parallel to the planar surface of the cover member, as shown in FIG. 1 of WO 02/15636. The floating diaphragm is free to move in its own plane, and thus can release its intrinsic stress, resulting very consistent mechanical compliance. However, this kind of “floating” diaphragm is required to be made of lower stress polysilicon, and the structure formation process is not compatible with CMOS process.
U.S. Pat. No. 7,346,178 discloses a microphone sensing element without dedicated backplate component. In the microphone sensing element, a movable diaphragm is supported at its edges or corners by mechanical springs that are anchored to a conductive substrate through rigid pads, as shown in FIGS. 1 and 2 of U.S. Pat. No. 7,346,178. In U.S. Pat. No. 7,346,178, the structure of the microphone sensing element is very simple, however, the diaphragm is required to be made of low stress polysilicon, and the substrate is required to be a low resistivity substrate, which is a standard substrate for formation of CMOS circuitry.
Patent document PCT/DE97/02740 discloses a miniaturized microphone. In the miniaturized microphone, an SOI substrate is used for formation of CMOS and the microphone backplate. However, the diaphragm is a polysilicon thin film formed in CMOS fabrication. Such a poly diaphragm normally has very high intrinsic stress which is difficult to control, thus resulting in unconsistent mechanical compliance.
U.S. Pat. No. 6,677,176 discloses a method for forming an integrated semiconductor device including a microphone and at least one MOSFET sensing transistor. In this method, the structure can be formed using CMOS thin films. However, it is difficult to control the intrinsic stress in CMOS thin films which may affect the device functionality and manufacturing yield.
In summary, most of prior arts are either incompatible with CMOS process or their structures have various inherent shortcomings in manufacturability.
Therefore, there is a need for a CMOS compatible MEMS microphone and method for manufacturing the same.